The invention relates to a television pick-up system particularly as employed in medical X-ray technology, comprising a television camera which contains a television camera tube of the vidicon type, which is provided with a signal plate (or electrode) with a photosensitive layer and a free surface of said layer, comprising devices for producing an electron beam as well as for deflecting and focusing the electron beam onto the signal plate, comprising means for adjusting the cathode potential of the television camera tube, and comprising an installation for processing the video signal picked off from the signal plate.
Television pick-up systems of this type are known wherein the potentials at the control grid and at the cathode of the television camera tube are constant. In television pick-up systems of this type, during the transmission of images with a great contrast ratio; for example, during the transmission of X-ray photographs, the difficulty which arises is that when the potentials are adjusted such that the bright locations can still be satisfactorily transmitted, the contrast of the dark picture locations is strongly reduced. For example, this problem occurs during the transmission of a thorax-radiograph which, in general, exhibits very bright lung fields (or areas). The transmission of the bright image locations can, indeed, be guaranteed; for example, in the case of an antimony trisulfide vidicon, by reducing the cathode potential and the potential at the control grid (grid no. 1) or by stopping down the optical aperture; however, as mentioned, this results in an undesirable decrease in the contrast of the dark image regions.
In television X-ray fluoroscopy, above all in the case of surgical television, the desire for a greater dynamic range of the television camera tube also exists. In the known television pick-up devices, namely, the contrast control of the X-ray television system is generally set relatively low during fluoroscopy, in order to still be able to effect a good transmission of subject-dependent signal peaks. However, the contrast of the dark image regions is thereby strongly reduced.